refactor: rename :core module group to :doom

I'll soon introduce a new family of core modules which will house most
of what's currently in the doom-*.el libs, moving them out of core.

lisp/doom-modules.el

@@ -216,7 +216,7 @@ Return its PROPERTY, if specified."
     (setq doom-modules (make-hash-table :test 'equal))
     ;; Register Doom's two virtual module categories, representing Doom's core
     ;; and the user's config; which are always enabled.
-    (doom-module--put '(:core . nil) :path doom-core-dir :depth -110)
+    (doom-module--put '(:doom . nil) :path doom-core-dir :depth -110)
     (doom-module--put '(:user . nil) :path doom-user-dir :depth '(-105 . 105))
     ;; DEPRECATED: I intend to phase out our internal usage of `use-package' and
     ;;   move it to a :config use-package module. The macro is far too complex
@@ -378,7 +378,7 @@ cdr. See `doom-module-put' for details about the :depth property."
 PATHS-OR-ALL can either be a non-nil value or a list of directories. If given a
 list of directories, return a list of module keys for all modules present
 underneath it.  If non-nil, return the same, but search `doom-module-load-path'
-(includes :core and :user). Modules that are enabled are sorted first by their
+(includes :doom and :user). Modules that are enabled are sorted first by their
 :depth, followed by disabled modules in lexicographical order (unless a :depth
 is specified in their .doommodule).
 
@@ -453,7 +453,7 @@ If ENABLED-ONLY?, return nil if the containing module isn't enabled."
                         (doom-module-active-p group name))
                     (cons group name))))
             ((string-match (concat "^" (regexp-quote doom-core-dir)) path)
-             (cons :core nil))
+             (cons :doom nil))
             ((string-match (concat "^" (regexp-quote doom-user-dir)) path)
              (cons :user nil))))))